1. Field of the Invention
The present invention relates to a method for making a nonwoven fabric by the heat treatment of a web comprising heat-adhesive composite fibers, in which sufficient bulk is achieved under such treating conditions that a pressure is applied to the web during the heat treatment.
2. Statement of the Prior Art
Heretofore, there has been known a method for making porous nonwoven fabrics by the heat treatment of a web, at least a part of which is composed of heat-adhesive composite fibers containing as the composite components fiber-formable polymers having different melting points, to head-bond the fibers together. Among others, the use of heat-adhesive composite fibers containing as the composite components polypropylene and other polymer having a lower melting point than that of the polypropylene has been known from long ago. With such heat-adhesive composite fibers, however, a problem arises that the bulk of a nonwoven fabric obtained therefrom is lower than that of the web before the heat treatment. This is because they are generally heat-bonded together with large shrinkage, since latent crimps are developed by the heat treatment in addition to the original three-dimensional crimps which have already been developed.
To solve such a problem, it has been known to anneal the heat-adhesive composite fibers prior to obtaining a nonwoven fabric therefrom, for the pre-development of latent crimps and, then, make a nonwoven fabric. In this case, however, it is difficult to control the number of crimps. In addition, the processability of the web and the bulk of the nonwoven fabric are largely affected by too large or small a total number of crimps after annealing. With such a method, therefore, difficulty is practically encountered in eliminating the above-mentioned problem.
Incidentally, Japanese Patent Laid-Open No. 58-23951 discloses a method for making bulky nonwoven fabrics, using heat-adhesive composite fibers having three-dimensional crimps but not substantially latent crimps, which are obtained by specifically limiting the Q value of polypropylene which is one of the composite components and stretching conditions. In the method disclosed, however, since the heat treatment is carried out with no application of any substantial pressure to the webs, the obtained nonwoven fabrics become bulky. With this method, however, it is impossible to obtain sufficiently bulky nonwoven fabrics, when a dryer of the type that applies pressure to webs at the time of heat treatment is used, such as a suction dryer which is now enjoying increasing use.